1. Field of the Invention
The present invention relates to an image processing method and image processing apparatus, and to an image forming apparatus comprising same, and more particularly, to an image processing method and image processing apparatus, and an image forming apparatus comprising same, whereby both image resolution and tonal gradation can be achieved simultaneously when forming an image by means of dots using a plurality of inks.
2. Description of the Related Art
In the related art, an image forming apparatus is known in which images are formed by outputting a plurality of colors onto a recording medium by means of a coloring material, such as ink, toner, or the like. In an image forming apparatus of this kind, images are formed by means of a plurality of dots formed by means of ink, toner, or the like. Therefore, an image is reproduced according to whether or not dots are present on a recording medium, such as white paper. Since the number of inks used is limited, conventionally, a half-toning method is known which is used to represent continuous tonal gradation by means of a limited number of inks.
On the other hand, in many cases, the image data inputted to an image forming apparatus is multiple-value image data. Therefore, when an image represented by multiple-value image data of this kind is outputted to an image forming apparatus such as that described above, it is necessary to convert the multiple-value image data into binary image data.
Conventionally, a technique such as error diffusion or dithering is used to convert multiple-value image data into binary image data, and various methods based on these techniques have been proposed for obtaining satisfactory binary image data.
For example, a method is known in which the spatial frequency or a characteristic quantity corresponding to same is sampled for each pixel of the input image, and an optimal error diffusion matrix is selected, if error diffusion is being used, or an optimal threshold value matrix is selected, if dithering is being used, in accordance with the characteristic quantity. In this way, suitable binary image data which satisfies human visual perception characteristics is obtained (see, for example, Japanese Patent Application Publication No. 8-214159).
However, Japanese Patent Application Publication No. 8-214159 provides no disclosure at all regarding methods for using a plurality of inks. The characteristics of human visual perception indicate high sensitivity to spatial frequency in respect of achromatic colors and low sensitivity to spatial frequency in respect of chromatic colors. Therefore, it is necessary to obtain a binary image which satisfies human visual perception in respect of a plurality of inks representing chromatic colors, in particular.
However, in the method described in Japanese Patent Application Publication No. 8-214159, there is no disclosure regarding use of a plurality of inks and although overlapping between a plurality of inks is controlled in conventional half-toning technology in order to improve color reproduction and granularity, no technology is currently known for controlling overlapping between a plurality of inks with the object of achieving compatibility of image resolution and tonal gradation.